Stenting rings on the market are made from biologically compatible metals such as stainless steel or nickel-titanium shape memory alloy and, as these materials are relatively poorly visible in x-ray images, they are often equipped with “markers” that are more opaque to x-rays thereby allowing the radiologist to monitor the position of the stent in a bodily lumen. It is advantageous when the radiopaque marker has an electrochemical potential similar to that of the stent metal, thereby to minimise electrochemical corrosion of the prosthesis that includes the stent and the marker. This is one reason why tantalum is a popular choice as radiopaque marker or nickel-titanium shape memory alloy stents.
In designing the marker, compromises are unavoidable. The bigger the marker, the more effectively it reveals to the radiologist the location of the stent. However, the bigger the marker, the more it can interfere with trans-luminal delivery of the stent and, indeed, performance of the stent at the stenting site. One way to accomplish an effective compromise is to provide more or less a complete ring of marker material around the stent lumen, but cantilevered from the stent metal as such, beyond each end of the stent cylinder. Such a prosthesis exhibits, after delivery and deployment, an expanded diameter cylindrical space in which the stent is working and, beyond each end of that stent cylinder, a plurality of radiopaque markers, attached to the stent, and spaced from each other around the circumference of the stenting cylinder. Within the length of the stenting cylinder, gaps between successive stent struts, as one advances around the circumference of the stenting cylinder, are relatively small. Conversely, when one advances around the circumference of the circle in which the spaced radiopaque markers are to be found, the gaps between adjacent markers around the circumference are relatively large.
The purpose of the markers is to inform those operating on the patient where exactly within the patient the stent is located. Markers located at positions cantilevered beyond the ends of the stent cylinder are not ideal, in that they are not precisely coincident with the ends of the stent cylinder.
For marker disclosures see, for example, WO-A-97/33534, WO-A-02/078762 and WO-A-03/101343 as well as EP-A-1212991 and 1356789.